The present invention relates to cellular and satellite communications. More particularly, the invention relates to a method and a system for providing signaling bursts for maintaining communications channel transmissions during periods of voice inactivity during ongoing voice communications between a transmitter and a receiver in a time division multiple access (TDMA) mobile satellite communication system.
A mobile satellite communication system such as the Geosynchronous Earth Orbit Mobile (GEM) network discussed herein, typically includes one or more satellites, at least one fixed ground terminal such as a gateway system (GS) and several mobile access terminals (ATs). The access terminals typically communicate with the public switched telephone network (PSTN) or other mobile terminals via an air communication interface between the satellite and the gateway. Using the mobile access terminals, the satellite system provides a variety of telephony services. Satellite telephony systems as described herein share call processing information with terrestrial systems such as the GSM cellular system to allow compatibility between the satellite, cellular, and the public switch telephone network services. The terrestrial standards such as GSM may not apply directly to the mobile satellite communication system, more particularly the satellite air interface poses physical constraints not accounted for in the GSM architecture.
A number of communication systems utilizing satellites and small mobile terminals provide voice and other information communication. In all such systems, the bandwidth and satellite power associated with the communication links may be expensive and wasteful of limited resources. In addition, the mobile access terminals such as hand-held terminals (HHTs), which are often small, hand-held devices, are constrained by power consumption and related battery life concerns.
In maintaining an active voice communications channel, however, information must be transmitted on a regular basis for synchronization between the satellite and the access terminal, e.g., for timing, frequency, and power parameters. During voice communications, periods of voice inactivity may occur approximately half of the time. Therefore, appropriate design of burst formats, combined with voice activity detection, may provide significant power reduction. A number of specific concerns are associated with the form of information communications necessary for maintaining a communications channel, including power control information transfer requirements, power level consistency in the presence of highly variable power amplifiers, background noise level and characteristic communication, support for frequency and timing parameter tracking, timeliness of information delivery, and robust communications.
Thus, there exists a need for a method and a system for performing Keep-Alive Burst (KAB) communications during periods of voice inactivity to maintain the integrity of the voice communication transmissions over a communications channel, and provide acceptable performance with a minimum amount of power being used by the satellite and access terminal systems.
In the following description, a satellite communications system is described. As will be appreciated by a skilled artist, however, the teachings of the present invention apply to many communications systems, not just satellite-based systems. Thus, references herein to satellite systems should be understood as being directed to specific embodiments, as opposed to the invention generally.
Accordingly, the present invention, in particular embodiments, addresses a key opportunity for power savings at both the satellite and the access terminals to limit transmission of significant power to those times when voice communications is active. During periods of silence, which typically occur about sixty percent of the time, much less power may be transmitted. Nonetheless, for a variety of reasons discussed herein, some power continues to be required for transmissions during periods of voice inactivity in the form of bursts that are transmitted during such periods to maintain the integrity of the communications channel. Information transmitted during voice inactivity by such keep-alive bursts (KABs) may be categorized into two types of information, namely, explicit digital information and information implicit in the waveforms transmitted. By adopting a burst format which accounts for the necessary explicit and implicit information required for transmission during keep-alive bursts, a combination of various features in terms of power modulation in burst format results in reduced power and delay, and improves performance when compared with conventional techniques.
Briefly summarized, the present invention relates a system and method employing an access terminal for maintaining discontinuous communications including a gateway receiver for receiving the discontinuous information, a radio frequency (RF) communication link via geosynchronous earth orbit satellite for conveying multiple communication channels using time division multiple access (TDMA), the access terminal initiating information communication with the receiver via at least one of the multiple communication channels. The access terminal further includes a memory for storing protocol processing information and a transmitter for establishing the radio frequency communication link to the receiver of the terrestrial gateway system. The access terminal memory provides for storing of a signal pattern or protocol assigned to the access terminal by the gateway receiver or transmission of keep-alive bursts by the transmitter during periods of inactivity to maintain information communication with the receiver.
It will be understood that both the foregoing and general description in the following detailed description are exemplary and intended to provide further explanation of the invention as claimed. The accompanying drawings provide an understanding of the invention as described in the preferred embodiments to illustrate the invention and serve to explain the principles of the invention.